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Sagging Rockers anyone?

Discussion in 'General Mustang Discussion' started by Mach1 Driver, Jul 11, 2018.

  1. Mach1 Driver

    Mach1 Driver Active Member

    A little background: since our Stangs are unibody and don’t have a chassis, the load is carried by a combination of components. The rear sub frames extend from the tail to an area under the rear seat. From there they are connected by the rear torque boxes outboard to the rocker panels. The rockers and floor bear the center load along the bottom of the car. If you have a coupe or fastback, some of this load is spread to the roof. If you have a convertible, an inner rocker is added back to back with the outer rocker to compensate for the absence of the roof. The rockers extend to the just behind the front tires, where the front torque boxes connect the load back inboard to the front sub frame.

    I’ve been investigating how to install convertible inner rockers in my fastback. Street or Track did a good article showing how to upgrade a coupe/fastback https://streetortrack.com/Installing-convertible-rockers-into-a-65-68-p-14.html, but they noted that they ran into a problem that is seemingly on virtually all unmodified cars. The inner rocker they used is a straight box approximately 2.5 x 5.75 x 75. They found that just behind the front torque box (about 10” from the front of the rocker), the outer rocker began to angle up and was about 1/2” too high at the front. Since their new inner convertible rocker was straight, it was off by 1/2” at the front and they had to take measures to weld it properly, since the pinch welds were no longer in a position to weld to.

    I put a straight edge under the pinch weld on my Mach and found the same condition- it angles up in the front 10” or so of the rockers. I have a southern California car where it rarely rains and was always garaged. She’s a survivor that has very little rust. You would be hard pressed to find a better preserved car. Here is my theory: I believe that the outer rockers on all our cars fatigue and bend just behind the front torque box. Think about it and it makes sense- most of the weight is in the front, and the weakest point would be right behind the torque box. Following that thought weight wise, I have a 351w with an FMX, and relatively low mileage on the body of the car at 73k. Suffice to say, that if mine does it, most of them do.

    So I’m wondering if anyone knows if NEW replacement outer rockers are straight along the lower pinch weld area? Pictures of parts appear to be straight. If so that pretty well proves that the cars fatigue and sag over time. What the heck, they are only 10 times past their expected design life.
  2. B67FSTB

    B67FSTB The NorCal dude from Belgium

    So , you say that the mustang will sag over time and that this happen at the first 10 inch of the rocker ?
    So , the first 10 inch is the part behind the front fender ?
    Then IMHO you have to see this at the alignment of the fenders /doors.
  3. Mach1 Driver

    Mach1 Driver Active Member

    That's a good point that I hadn't considered, and i hope I'm wrong, but I'd feel better if someone could put a straight edge across the bottom pinch weld of a new rocker.
  4. Midlife

    Midlife Well-Known Member Staff Member Moderator

    It is not a new rocker that you want to measure, but a genuine NOS rocker from the original production line. Reproduction rockers are more likely than not to be modeled after a unit taken from a car that has been in service and may exhibit this problem.

    Personally, I think you're off track and that the original design has a slight bend to it. As others have said, the fender/door area would show the effects of this fatigue.
  5. Horseplay

    Horseplay I Don't Care. Do you?

    There is also the trans tunnel that runs the length from firewall (fore of the point you specify) to the sheetmetal over the rear end/rear frame rails which is extremely solid. If such a pronounced "sag" were to occur there would also have to be distortion somewhere in the floor metal to compensate for the shortening of length. The tunnel itself would have to exhibit a distinct ridge or push into the firewall or rear metal. I can tell you, anecdotally, that in my car there is nothing in these areas and yet the front of my rockers (as determined with a four foot level) does have a slight upward slope. Also, as a unibody, the roof structure would also have to shift if the lower section were able to move as suggested, unless welds failed allowing the lower to separate from the upper body.
  6. Mach1 Driver

    Mach1 Driver Active Member

    Also a good point. In hindsight I also think the door gap would be an issue. I'm waiting for a reply on another forum from a particular restoration guy that should have the answer. I'll let you know.

    KBMWRS What did the moron say today?

    Get subframe connectors

    Or using what was used in the day your purpose?
  8. msell66

    msell66 Burning Fossil Fuels at c2 Donator

    I read the thread title and heard "Sagging Knockers".
  9. Midlife

    Midlife Well-Known Member Staff Member Moderator

    I thought it was about sagging hippies from the 60's still listening to Jefferson Airplane...
  10. Mach1 Driver

    Mach1 Driver Active Member

    I have heard from two respected chassis guys on another forum and they confirm that the rockers are not sagging but built that way. One says he has built templates for the next time he does an inner convertible rocker installation and needs to modify the parts to fit- he pie cuts it near the front and welds it back together and adds material along the top pinch weld in front. He says he allowed for a 3/4" rise in the front 19" of the inner rocker. Case closed, theory rejected.
    tarafied1 likes this.
  11. Fst Blk

    Fst Blk Well-Known Member

    Nice investigation work. I liked your hypothesis but saw flaws that were pointed out.

    Sent from my iPhone using Tapatalk
  12. Mach1 Driver

    Mach1 Driver Active Member

    This is from a discussion On VMF about SFCs. Patrick is a mechanical engineer:

    The difference between the two (SFCs and inner rockers) are pretty drastic. You have to consider the section modulus (generally the resistance of a shape to bending) and the inertial moment (generally the resistance of a shape to twisting) between the two shapes. Comparing a 2"x2"x.125" square tube to a 3"x6"x.06" (guessing at rough size here) 'vert inner rocker it looks like this:

    Square Tube
    Section Modulus = .55 in all directions
    Inertial Moment = .55 in all directions

    'Vert Inner Rocker
    S(x) = 1.81 S(y) = 1.24
    I(x) = 5.42 I(y) = 1.87

    In reality, the differences may be more substantial than this with the 'vert inner rockers as they are essentially the mirror opposite of the outer rockers all welded together. I believe (and I may be wrong) the factory install of the 'vert inners was a only a outer and inner section welded together with one vertical divider wall. What I did on my car is weld in the full inner rocker with its own vertical wall to the already existing outer rocker with its own vertical wall in place. My car effectively has complete rocker assemblies welded back to back with spot welds top and bottom and full stitch weld along the top edge of where the two rockers meet.

    So from the above, I imagine the reason Ford added inner rockers is to keep the car from breaking in half length wise...since there is no roof structure to perform that function. From the numbers above, you can clearly see that simply adding a pseudo full frame via sub-frame connectors would most likely not prevent this type of failure. Second, I imagine the reason Ford added the inner one piece seat pan AND the outer seat pan boxes WITH solid plate tying the two together at the hump is to alleviate twisting...again, no roof to perform that function.

    Now apply these principles or thoughts if you will to hardtops or fastbacks. Yes they have a roof to perform the functions of alleviating longitudinal bending and twisting. However, only enough to keep the car from failing. Ok, so lets add some sub-frame connectors. Again, using the numbers above, you can clearly see they do not and will not perform the same task as adding inner rockers. Now lets add cross bars between the sub-frame connectors. Ok, we have maybe eliminated some small amount of twisting. Visually if you will, now imagine instead of cross bars of say 1 3/4" x 1/8" wall (guessing here) which has a section modulus of .24, we add all of the 'vert inner and outer seat pans and 'vert inner rockers. We have already proven via math, the sub-frame connectors pale in comparison to 'vert inner rockers. So compare the cross sectional area (more cross sectional area...higher section modulus...less movement) of the inner and outer seat pans that span the entire width of the floor and weld to the entire width of the floor top and bottom and weld to the 'vert inner rockers. You can see you have effectively created a huge boxed in inner and outer structure that is welded side to side, top to bottom, and inner and outer versus simply tying the front and rear frame rails together which do very little to prevent twisting or adding two small tubes side to side which also do very little to prevent twisting.

    I'm not saying the sub-frame connectors or cross bars do nothing. I am saying they do very little in comparison to other methods. I will also admit that adding the 'vert parts with torque boxes may not be for everyone. It is a very invasive and time consuming project. Not to mention you do have to deal with the hump over the hump and use some of the 'vert interior pieces. This may also not be for everyone.

    These are just my thoughts...they may not be considered accurate by some or all. However, I also believe if Ford could have gotten away with simply adding sub-frame connectors and cross bars, they probably would have because it is a hell of lot faster and less expensive.
  13. Horseplay

    Horseplay I Don't Care. Do you?

    FYI -- He also hangs out here when he feels the need to be with the cooler crowd. :cool:
  14. Horseplay

    Horseplay I Don't Care. Do you?

    A couple needed points in response to Patrick's explanation about SFC and convertible inners. Not all SFC are created equal. There are some made of bent/formed sheetmetal. Others are made up of round steel tubing while yet again some use varying thicknesses of square steel tubing. Then you have to consider how the SFC are affixed to the car. Some merely bolt on. :eek:. Others might be just welded fore and aft. :( Then some are welded along the full length of the underbody. :) And lastly, some are even fitted into the existing frame rails and then fully welded into place the full length, tying them into the rocker and floor structure as one. :D I am clearly not an engineer but would suggest that the last style of connector and installation practice would certainly compare well to the inner rocker piece. The strength of the convertible piece comes as much from its installation as the metal structure itself, does it not?

    I argue in favor of a good set of SFC, installed via full welding, over the inners simply because, to me, it is much less intrusive to the driver compartment. They perform the desired duty (strengthening of the chassis) more than adequately. At least in the cases of coupes and fastbacks. I should note that torque boxes are also required to achieve good results.
  15. msell66

    msell66 Burning Fossil Fuels at c2 Donator

    Patrick is a putz.

  16. Mach1 Driver

    Mach1 Driver Active Member

    Mark, I see no need to disparage Patrick, he presented engineering data in a clear and concise manner. Kudos to Patrick.

    Terry, he was discussing a particular SFC, I didn't include all the information from multiple posts. I'd really rather not get in an discussion on the merits of this or that SFC. Most of them will not have the cross sectional area or wall thickness to rival an inner rocker- that's why Ford used inner rockers on verts. Also, if you use SFCs they get in the way of side pipes. I like inner rockers. If you want to run the math and SFCs stack up well against inner rockers then good- go for it. The only test I've ever seen used one particular style SFC (don't remember which) and it didn't do anything against inertial moment- nothing. The body still twisted as much. Testing showed that you would be much better off installing an export brace and a metal sheet behind the rear seat.

    Peace! brothers- that's all I have to add.
  17. KBMWRS

    KBMWRS What did the moron say today?

    Now this is what I like. Someone actually investigating the engineering and requirements rather than just slapping something together. Hope it works out for you.
    Mach1 Driver likes this.
  18. Horseplay

    Horseplay I Don't Care. Do you?

    Mark is just a big, ugly, mean and nasty person. He hates everyone because deep down inside hates himself. I think it stems from a severe lack of confidence derived from decades of humiliation justifiably bestowed upon him for sporting those wings he has instead of ears.
  19. Horseplay

    Horseplay I Don't Care. Do you?

    It's anecdotal for sure but I can vouch that my current car chassis feels and acts a TON better than any other '65 I have owned or driven. My SFCs are thick walled square tubing welded along their length to the floor and rails which is almost the same as how an inner would affix short of being in the same plane and back to back with the standard rockers. I can see how they fall short of delivering on every angle to the degree the inners would provide but I honestly don't think that deficit will ever matter. When I put some "stickies" on the back at the track I'll know for sure. If that front wheel comes up and the windshield doesn't crack or pop out I'll consider myself vindicated. ;)
    Mach1 Driver likes this.
  20. msell66

    msell66 Burning Fossil Fuels at c2 Donator


    Patrick knows I’m just play’n.


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